The invention relates generally to the field of electrical superconductors and more particularly to a method of manufacturing an insulated superconductor where the superconductive characteristics are formed in situ by prolonged heat treatment.
Many metals and metallic compounds exhibit superconductivity at very low temperatures but only certain of these are suitable for use in high-magnetic field applications such as very powerful generators or accelerators. One material found useful for high field applications in the intermetallic compound Nb.sub.3 Sn. Nb.sub.3 Sn is however very brittle and a conductor containing Nb.sub.3 Sn filaments cannot be mechanically worked or even wound into a small radius coil without breaking such filaments.
One approach to solving this problem has been to make flat tape-type conductors in which the Nb.sub.3 Sn layer is so thin it can tolerate some amount of bending. The current-carrying capability of these superconductors is low due to the low density of superconducting material.
Another approach to overcoming the problem is to make superconductors with a plurality of fine Nb.sub.3 Sn filaments in a matrix material. As is now well known in the art such conductors are made by forming Nb filaments in a CuSn matrix, and after the conductor has been worked to final size, heat treating it to react the Nb and Sn to form in situ filaments of the superconductive intermetallic compound Nb.sub.3 Sn. This final reactive heat treatment is generally carried out at a temperature of the order of 700.degree. C or more for a period of up to several days.
In many cases it is desirable, if not mandatory, to insulate the conductor before subjecting it to the final reaction heat treatment. This might be done just to avoid the additional handling necessary to insulate it after the in situ formation of the brittle Nb.sub.3 Sn phase, or it might be dictated by the fact that the reaction heat treatment is carried out after the conductor is formed into a braid or cable or wound into a magnet coil or other device in which the successive turns must be insulated from one another. Whatever the reason, very few insulating materials that can be applied to the conductor can retain their insulating capability through the high temperatures and long times of the reaction heat treatment.
In view of the foregoing difficulties, it is the principal object of the invention to provide a method for making an insulated superconductor where the insulation can withstand prolonged high-temperature heat treatment.
It is another object of the invention to provide an insulated superconductor capable of withstanding prolonged high-temperature heat treatment.
The invention contemplates lining the inner walls of the final extrusion can of the precursor billet with a layer of a valve metal such as tantalum. The lined can is then filled with a plurality of bars, some of which have the required chemical composition for forming a superconductor such as Nb.sub.3 Sn when heat treated, and the others are made of normally conductive material to stabilize the composite superconductor. In some instances, the superconductor bars comprise compositions such as NbTi that need no further treatment.
The filled can is then extruded and drawn until a conductor of the desired size is produced. The ductile material of the can is then removed, preferably by dissolving it in acid such as nitric or sulphuric acid, to expose the underlying layer of valve metal. The conductor is then anodized to oxidize a sufficient amount of the valve metal to form a refractory non-conductive oxide on the conductor. The valve metal oxide is sufficiently durable that it can withstand being formed into a final product as well as withstand the prolonged high-temperature in situ heat treatment required to form the Nb.sub.3 Sn filaments within the conductor.